dodgr: Distances on Directed Graphs

Distances on dual-weighted directed graphs using priority-queue
shortest paths (Padgham (2019) <doi:10.32866/6945>). Weighted directed
graphs have weights from A to B which may differ from those from B to A.
Dual-weighted directed graphs have two sets of such weights. A canonical
example is a street network to be used for routing in which routes are
calculated by weighting distances according to the type of way and mode of
transport, yet lengths of routes must be calculated from direct distances.